Several IPv6 transition mechanisms require the usage of stateless or stateful translators (commonly named as NAT64) able to allow IP/ICMP communication between IPv4 and IPv6 networks.

Those translators are using either a default Well-Known Prefix (WKP), and/or one or several additional Network Specific Prefix (NSP), which need to be configured into the nodes willing to use the translator. Different translators will likely have different IPv6 prefixes, to attract traffic to the correct translator. Thus, an automatic translator prefix discovery method is necessary.

This document defines a DHCPv6-based method to inform DHCPv6 clients the set of IPv6 and IPv4 prefixes it serves. This DHCPv6 option can be used by several transition mechanisms such as SIIT, 464XLAT, EAM.

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464XLAT[RFC6877] describes an IPv4-over-IPv6 solution as one technique for IPv4 service extension and encouragement of IPv6 deployment. The 464XLAT architecture uses IPv4/IPv6 translation, described in [RFC6144], and standardized in [RFC6052], [RFC7915], and [RFC6146]. It encourages the IPv6 transition by making IPv4 service reachable across IPv6-only networks and providing IPv6 and IPv4 connectivity to single-stack IPv4 or IPv6 servers and peers. In the 464XLAT architecture, the CLAT (customer-side NAT46 translator) must determine which of potentially several PLAT (provider-side NAT64 translator) IPv6 prefix to use in order to send a packet to the PLAT with connectivity to its destination.

[RFC7050] describes a mechanism to learn the PLAT-side IPv6 prefix for protocol translation by DNS64[RFC6147]. Although it supports multiple PLAT-side prefix by responding with multiple AAAA records to a DNS64 query, it does not support mapping IPv4 prefixes to IPv6 prefix, which would be required, for example, if one PLAT has connectivity to the general Internet following a default route, another has connectivity to a BGP peer, and a third has connectivity to a network using private addressing[RFC1918]. Therefore, in the scenario with multiple PLATs, [RFC7050] does not directly support destination-based IPv4 routing among PLATs; instead, the DNS64 database must contain equivalent information. It also requires the additional deployment of DNS64 service in customer-side networks, which is not required in 464XLAT deployment. Indeed, this scenario, which may become very common in wired access networks, has not even been considered by [RFC7051].

This document proposes a method for the translator (NAT64) IPv6 prefix discovery based on DHCPv6, which is widely deployed and supported in customer networks. It defines two new DHCPv6 options for use by a DHCPv6 client to discover the translator IPv6 prefix(es). Also, the proposed mechanism can deal with the scenario with multiple independent DNS64 databases supporting separate translators.

The NAT64 Prefix Option is encapsulated in the NAT64 Prefix List Option. This option allows the mapping of destination IPv4 address ranges (contained in the IPv4 Prefix List) to a NAT64 IPv6 prefix. If there is more than one such prefix, each prefix comes in its own option, with its associated IPv4 prefix list. In this way, the DHCPv6 client can select the NAT64 with the corresponding destination IPv4 address.

NAT64-prefix: The NAT64 IPv6 prefix to be used by the DHCPv6 client for the IPv6 address synthesis. MUST follow the specifications of [RFC6052].

NAT64-suffix: The NAT64 suffix to be used by the DHCPv6 client for the IPv6 address synthesis, specified in Section 2.2 of [RFC6052]. The length of this field, in octets is 12 - NAT64-prelen. Not used in case of using the WKP (i.e., 64:ff9b::/96) or any other NSP (Network-Specific Prefix) which a length of 96 bits (/96).

IPv4 Prefix List: This is an optional field. The format of the IPv4 Prefix List is shown in Figure 3. It is a list of zero or more IPv4 Prefixes. Each entry is formed by IPv4-prelen and IPv4 Prefix. The total length of the field is 5*number of IPv4 prefixes.

The client requests the OPTION_NAT64_PREFIX_LIST option using the Option Request option (ORO) in every Solicit, Request, Renew, Rebind, and Information-request message. The NAT64-Type field defines the mechanism being used. If the DHCPv6 server includes the OPTION_NAT64_PREFIX_LIST option in its response, the DHCPv6 client may use the contained NAT64-prefix to translate the destination IPv4 address into the destination IPv6 address.

When receiving the OPTION_NAT64_PREFIX option with IPv4 Prefix List, the DHCPv6 client MUST record the received IPv6 prefix and the corresponding IPv4 prefixes in IPv4 Prefix List. When receiving the OPTION_NAT64_PREFIX option without IPv4 Prefix List, the DHCPv6 client MUST treat the IPv6 prefix and the default IPv4 prefix 0.0.0.0/0 as one of the records.

If the DHCPv6 client loses contact with the DHCPv6 server, the DHCPv6 client SHOULD clear the prefix(es) it learned from the DHCPv6 server.

When translating the destination IPv4 address into the destination IPv6 address, DHCPv6 client MUST search an IPv4 routing database using the longest-match-first rule and select the IPv6 prefix offering that IPv4 prefix.

In order to make sure that the client has the right information, only a single container with all the current NAT64 Prefix List Option is allowed. When a new one is received, the client MUST clear the previous NAT64 Prefix List Option information and use only the new one.

The figure below shows an example of message flow for a Client learning IPv6 prefixes using DHCPv6.

In this example, two IPv6 prefixes are provided by the DHCPv6 server. The first IPv6 prefix is 2001:db8:122:300::/56, the corresponding IPv4 prefixes are 192.0.2.0/24 and 198.51.100.0/24. The second IPv6 prefix is 2001:db8:122::/48, the corresponding IPv4 prefix is 192.0.2.128/25.

When the DHCPv6 client receives the packet with destination IPv4 address 192.0.2.1, according to the rule of longest prefix match, the NAT64 with IPv6 prefix 2001:db8:122::/48 is chosen. In the same way, the NAT64 with IPv6 prefix 2001:db8:122::/48 is chosen.

Considerations for security in this type of environment are primarily around the operation of the DHCPv6 protocol and the databases it uses.

In the DHCPv6 server, should the database be compromised, it will deliver incorrect data to its DHCPv6 clients. In the DHCPv6 client, should its database be compromised by attack or polluted by an incorrect DHCPv6 server database, it will route data incorrectly. In both cases, the security of the systems and their databases in an operational matter, not managed by protocol.

However, the operation of the DHCPv6 protocol itself is also required to be correct - the server and its clients must recognize valid requests and reject invalid ones. Therefore, DHCPv6 exchanges MUST be secured as described in [RFC3315].

We request that IANA allocate two DHCPv6 option codes for use by OPTION_V6_PLATPREFIX_LIST and OPTION_V6_PLATPREFIX from the "Option Codes" table. Similarly, a request to IANA for assigning the NAT64-Type field codes. The following initial values are assigned in this document (values are 16-bit unsigned intergers).